The most frequently used style manual for citing to Legal Documents is the The Bluebook : a uniform system of citation. APA, MLA and Chicago Manual of Style all refer to the Bluebook for citing to certain documents such as cases.
See below for more details about citing to legal documents.
A legal citation is a reference to a legal document such as a case, statute, law review article, etc.
Most legal citations consist of the name of the document (case, statute, law review article), an abbreviation for the legal series, and the date.
The abbreviation for the legal series usually appears as a number followed by the abbreviated name of the series and ends in another number.
For example: Morse v. Frederick, 551 U.S. 393 (2007) is a citation to a Supreme Court case which can be found in volume 551 of the United States Reports (U.S.) beginning on page 393.